The nation marked a decade since the worst terrorist attacks in U.S. history with official ceremonies and private tributes Sunday to the thousands who perished inside — or protecting — the symbols of American power on Sept. 11, 2001.

Political and religious leaders urged the nation to recall the spirit of common purpose that defined that day and its immediate aftermath but which has faded amid the acrimonious debate over America’s response to the attacks and the effect that response has had on the nation’s politics, finances and values.

Around the world, U.S. troops marked the anniversary in small, solemn ceremonies, while a number of countries expressed solidarity with the United States in the enduring fight against terrorism. Star-spangled ribbons were visible nearly everywhere — from survivors’ lapels to the jerseys of NFL players on the season’s opening weekend.

Remembrances ranged from the gravely hopeful, delivered by a political cross-section of elected leaders, to the heartbreakingly intimate — a surviving son’s regret for never having received his father’s advice on how to ask a girl on a date, a daughter’s teary message to her father that she would never forget him.

But amid the recollections were reminders that the war that began suddenly a decade ago continues. The Afghan Taliban, whose government harbored al-Qaeda before the attacks and was overthrown by the U.S. invasion that followed, rammed a truck packed with explosives into a military base in eastern Afghanistan, wounding nearly 80 NATO soldiers.

Security in New York City and Washington remained heightened in light of the threat of a vehicle-bomb attack on the anniversary. An FBI official said Sunday that law enforcement agencies had not discounted the threat, which continued to be investigated.

Beginning in Lower Manhattan, President Obama and first lady Michelle Obama paid respects at each of the attack sites, in a long day of mostly silent, symbolic tribute. At the culminating event at the John F. Kennedy Center for the Performing Arts, Obama called on the country to “honor those who have been lost” and “rededicate ourselves to the ideals that define our nation.”

“In the decade since, much has changed for Americans,” he said. “We’ve known war and recession, passionate debates and political divides. We can never get back the lives we lost on that day, or the Americans who made the ultimate sacrifice in the wars that followed.”

“Yet today, it is worth remembering what has not changed,” he continued. “Our character as a nation has not changed. Our faith — in God and each other — that has not changed.”

Ground Zero

At Ground Zero, where it began a decade ago, thousands of relatives of those who died in the World Trade Center towers turned out under clear skies that clouded as the memorial program began.

The ceremony connected six prolonged silences — commemorating the moments, a decade ago, of the plane crashes in Lower Manhattan, at the Pentagon and in a field on the outskirts of Shanksville, Pa., and the collapse of the towers, 29 minutes apart, that forever altered America’s skyline and comfortable sense of its place in the world.

A bell chimed before each silence. In between, family members read the names of the 2,749 people who died in the towers. A memorial with those names etched deep in stone was revealed at the site for the first time.

At the ceremony, Obama appeared with his Republican predecessor, George W. Bush, whom he has praised and criticized for his leadership after the attacks and in the subsequent wars that have cost more than 6,200 American lives.

After the first moment of silence at 8:46 a.m., when American Airlines Flight 11 slammed into the North Tower, Obama read Psalm 46, telling a subdued audience, “God is our refuge and our strength.”

Bush followed 19 minutes later, when a decade ago United Airlines Flight 175 hit the South Tower, with a reading of Abraham Lincoln’s letter to Mrs. Bixby, who lost five sons in the Civil War. The letter concludes with Lincoln noting “the solemn pride that must be yours to have laid so costly a sacrifice upon the altar of freedom.”

Widows and orphans, fathers and sons, mothers and daughters paraded images of those they lost — on placards and T-shirts, buttons and tattoos.

“I love you daddy,” read one card, bearing his photograph.

Elaine Barrett, 49, who escaped her 92nd-floor office at Aon Risk Services in the South Tower, said she has been wracked by guilt for a decade. Her sister, Donna Giordano, who worked on the 103rd floor, died in the tower. Her remains have never been recovered.

“I’m here today because she is here,” she said. “I’ve been here every year, and I’ll be here every year until I can’t walk.”

Shanksville, Pa.

At the new national park near Shanksville established as a memorial to the 40 passengers and crew members who died on United Airlines Flight 93, the Obamas laid a wreath of white flowers at the marble wall bearing the names of the dead.

Those names were read aloud, a bell chiming twice after each. A children’s choir from nearby Johnstown sang the national anthem. Wally Miller, the county coroner who recovered remains from the crash site a decade ago, read a litany.

Many in the crowd reflected on how the attacks had shaken their perceptions of their country.

“We’ve realized that we’re not seen as benevolent leaders around the world,” said Norman Simard, a human resources manager from Indianapolis who extended a business trip to Pennsylvania to attend the ceremony. “We need to find a way not to be aggressors even though we still need to protect our interests.”

Washington

At the Pentagon, the sun shone as it did that morning a decade ago when, minutes after taking off from Dulles International Airport, American Airlines Flight 77 slammed into the building. More than 1,600 people gathered Sunday to remember the 184 civilians and service members who died in a sudden war.

With an enormous U.S. flag draped over the point of impact on the Pentagon’s formidable wall, Vice President Biden remembered the attack as a “declaration of war by stateless actors bent on changing our way of life.” The goal: “To break us.”

But, he said, “they did not know us.”

Instead, the attacks galvanized a “new generation of patriots — the 9/11 generation,” Biden said, noting that since then, nearly 3 million men and women have signed up for armed-services duty “to finish a war begun here that day.”

The ceremony ended with service members placing wreaths at every bench in the memorial, one for each life lost.

“That was very, very moving for me,” said Lisa Dolan of Alexandria, whose husband, Navy Capt. Robert Dolan, was killed in the Pentagon.

The Dolans’ daughter, Rebecca, said her father would have been honored by the ceremony, particularly the wreath-laying.

Obama placed a wreath at the memorial after returning from Shanksville.

On Sunday evening, the president wrapped up the day at a “Concert for Hope” at the Kennedy Center.

“These 10 years have shown that we hold fast to our freedoms,” he told a crowd of about 2,000. “Debates — about war and peace, about security and civil liberties — have often been fierce. But it is precisely the rigor of these debates, and our ability to resolve them in a way that honors our values, that is a measure of our strength.”

After the event — which featured singers Patti LaBelle, Alan Jackson and Denyce Graves — attendees left smiling and hugging, although with a touch of somberness.

Durriya Badani of Sandy Spring described the tone of the event as a combination of “solemnity and hope.”

The world

Remembrances worldwide reflected the mixed legacy of America’s military response to the attacks.

In a statement Sunday, the government of Pakistan said it “joins the people of the United States and of the world in honoring the memory of all those who lost their lives on 11 September, as well as those who have been victims of terrorism around the world.”

Pakistan is a key, if unpredictable, ally in the U.S. war effort in Afghanistan. In the days before the anniversary, the Pakistani media emphasized the widely held perception that the country’s alliance with the United States has been harmful to Pakistan’s interests.

The News, an English-language daily, noted that “there will be no memorial for those innocents who have died in drone strikes” carried out by the CIA.

In London, British Prime Minister David Cameron, Prince Charles and U.S. Ambassador Louis Susman laid a wreath at a 9/11 memorial garden near the embassy grounds. Relatives of the 67 British citizens who died in the attacks read their names and placed a white rose for each on the memorial.

“To say that we understand, that we sympathize, that we hold you in our thoughts and prayers is true,” Prince Charles told the guests, “but I know it’s hopelessly, utterly inadequate.”

Staff writers Colum Lynch in New York, Shyamantha Asokan in Shanksville, and Michael Rosenwald, Dana Hedgpeth, Amy Gardner and Peter Finn in Washington, and correspondents Will Englund in Moscow, Ernesto Londono in Kabul, Karin Brulliard in Islamabad and Karla Adam in London contributed to this report.

Scott Wilson is the chief White House correspondent for the Washington Post. Previously, he was the paper’s deputy Assistant Managing Editor/Foreign News after serving as a correspondent in Latin America and in the Middle East.

David Nakamura covers the White House. He has previously covered sports, education and city government and reported from Afghanistan, Pakistan and Japan.

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